This invention relates generally to ceramic insulation and, more particularly, to rigidized refractory fibrous ceramic insulation i.e., (xe2x80x9cfiberformxe2x80x9d or xe2x80x9cmicroformxe2x80x9d) for use in aerospace systems and methods for manufacturing such insulation.
Various components of aerospace systems must be insulated because they may be exposed to elevated service temperatures or large temperature gradients. Ceramic insulation is applied to aircraft engine and engine exhaust components such as nose cones, firewalls and exhaust ducts to protect them and surrounding structure. Ceramic insulation is also applied to the outer surfaces of space vehicles (i.e., xe2x80x9cspace shuttle tilesxe2x80x9d) to protect the vehicle against the elevated temperatures and significant temperature gradients experienced during takeoff and re-entry.
Bending, U.S. Pat. 5,041,321, describes a method of making rigid, low-density, fibrous ceramic insulation by forming a slurry of ceramic fibers; molding the slurry to form a soft felt mat; drying the mat; incrementally introducing a sol into the mat; and gelling the sol upon each infusion. The incremental addition of the sol is accomplished through a multiple impregnation or infusion technique in which a small amount of sol is initially infiltrated into the mat, is gelled, and is cured to stabilize the mat dimensionally. Stabilizing allows handling and further processing of the mat. The partially completed mat is strengthened to its final form by infusing, gelling, and curing addition sol in the mat. Usually several infusion cycles are required to achieve adequate strength. Each infusion increases the density of the mat. This technique cures the mat to a rigid shape without appreciable shrinking of the resultant structure. Density of the final product can be controlled. Typical densities are 8-25 1b/ft3.
Despite its many advantages, the method disposed in U.S. Pat. No. 5,041,321 has difficulty repeatedly producing relatively thick insulation products that have uniform structure and density. To be practical, thick insulation products should be relatively free of cracks, but cracks are a common problem with the method of U.S. Pat. No. 5,041,321. Therefore, processing improvements are desirable to improve the quality of these products and to reduce their cost of manufacture.
One aspect of the present invention is a thick fibrous ceramic insulation with consistently reproducible properties that could not consistently be made using previous methods. In another aspect, the invention is a method which more consistently produces crack-free insulation having more uniform structure and density and improved strength. The present method permits an increased forming rate and maintains a uniform distribution of constituents in the slurry (and the insulation), although its constituents may have different densities. Metal particles can be included in the slurry. The metal particles are oxidized or nitrided in-situ to form a refractory binder or to enhance the sol binder. The refractory metal oxides or nitride reaction products augment or replace ceramic whiskers often included in the slurry.
Briefly, the invention is directed to a process for forming ceramic insulation and the product obtainable by that process. The steps of the process include forming a slurry of ceramic fibers and/or microparticles and/or metal particles; molding the slurry to form a soft felt mat; impregnating the mat with a sol prior to drying the mat; gelling the sol to form sol-gel glass binder bonds so that the mat is dimensionally stabilized; and drying the mat to produce the desired ceramic insulation product. If metal particles are used, there is an oxidation or nitriding step to convert the metal to a refractory ceramic product.
The invention is also directed to a process including adding a dispersant and/or a flocculant to the slurry and, otherwise, completing the ceramic insulation as described.
The invention is further directed to a process in which the slurry is formed and molded to form a mat followed by infusing a catalyst into the undried mat to initiate gelling of the sol to its sol-gel binder state. The infused mat, then, is dried to produce the desired ceramic insulation.
The invention is still further directed to a process for forming a mat from the slurry by injecting the slurry into a mold with sufficient back pressure to force a portion of the liquid out of the slurry through a porous surface of the mold and to leave a mat on the porous surface.
A further aspect of the invention is a slurry used in the processes for forming ceramic insulation. The slurry includes ceramic fibers and/or microparticles and/or metal particles, water or another appropriate carrier fluid, and, optionally, a dispersant, a flocculant, or both.
Finally, the invention is directed to ceramic insulation having consistent microstructure and improved strength. The low-density insulation includes ceramic fibers, microparticles, or mixtures thereof and a sol-gel binder which is fully gelled through the entire thickness of the insulation. The methods of the present invention permit the reproducible preparation of thick sections without serious cracks or other defects.